This proposal is to request support for a Keystone Symposia meeting entitled ?Leukocyte Trafficking?,[unreadable] organized by Ulrich von Andrian, Federica Sallusto and Klaus Ley, which will be held in Keystone, Colorado[unreadable] from January 13 ? 18, 2008. Remarkable progress has been made in understanding the cellular and[unreadable] molecular mechanisms that govern the intravascular recruitment, extravascular migration and functional[unreadable] regulation of immune cells by visualizing cell-cell interactions using powerful intravital microscopy and flow[unreadable] cytometry techniques. The identification of peptide- and lipid-based regulators of immune cell entry and exit[unreadable] and the structure-function relationship of leukocyte adhesion molecules at atomic resolution have resulted in a[unreadable] refined understanding of how leukocyte subset differentiation modifies migratory ability. Early clinical results[unreadable] with drugs such as natalizumab, which interferes with leukocyte trafficking, have demonstrated not only[unreadable] marked therapeutic efficacy in autoimmune diseases, but also potentially fatal immunosuppression that[unreadable] remains poorly understood. This meeting will bring together experts from a variety of fields to present and[unreadable] synthesize our current mechanistic insights into the regulation of protective and pathological immune[unreadable] responses and to determine how this knowledge can be translated to better and safer therapies.